Nuclear receptors are integrators of hormonal and nutritional signals, mediating changes to metabolic pathways within the body. Given that modulation of lipid and glucose metabolism has been linked to diseases including type 2 diabetes, obesity and atherosclerosis, a greater understanding of pathways that regulate metabolism in physiology and disease is crucial. The liver X receptors (LXRs) and the farnesoid X receptors (FXRs) are activated by oxysterols and bile acids, respectively. Mounting evidence indicates that these nuclear receptors have essential roles, not only in the regulation of cholesterol and bile acid metabolism but also in the integration of sterol, fatty acid and glucose metabolism.
Gpihbp1-deficient mice (Gpihbp1−/−) lack the ability to transport lipoprotein lipase to the capillary lumen, resulting in mislocalization of LPL within tissues, defective lipolysis of triglyceride-rich lipoproteins, and chylomicronemia. We asked whether GPIHBP1 deficiency and mislocalization of catalytically active LPL would alter the composition of triglycerides in adipose tissue or perturb the expression of lipid biosynthetic genes. We also asked whether perturbations in adipose tissue composition and gene expression, if they occur, would be accompanied by reciprocal metabolic changes in the liver.
Methods and Results
The chylomicronemia in Gpihbp1−/− mice was associated with reduced levels of essential fatty acids in adipose tissue triglycerides and increased expression of lipid biosynthetic genes. The liver exhibited the opposite changes—increased levels of essential fatty acids in triglycerides and reduced expression of lipid biosynthetic genes.
Defective lipolysis in Gpihbp1−/− mice causes reciprocal metabolic perturbations in adipose tissue and liver. In adipose tissue, the essential fatty acid content of triglycerides is reduced and lipid biosynthetic gene expression is increased, while the opposite changes occur in the liver.
lipoprotein lipase; hypertriglyceridemia; lipolysis; essential fatty acids; lipid biosynthetic genes
Sexual dimorphism in body weight, fat distribution, and metabolic disease has been attributed largely to differential effects of male and female gonadal hormones. Here, we report that the number of X chromosomes within cells also contributes to these sex differences. We employed a unique mouse model, known as the “four core genotypes,” to distinguish between effects of gonadal sex (testes or ovaries) and sex chromosomes (XX or XY). With this model, we produced gonadal male and female mice carrying XX or XY sex chromosome complements. Mice were gonadectomized to remove the acute effects of gonadal hormones and to uncover effects of sex chromosome complement on obesity. Mice with XX sex chromosomes (relative to XY), regardless of their type of gonad, had up to 2-fold increased adiposity and greater food intake during daylight hours, when mice are normally inactive. Mice with two X chromosomes also had accelerated weight gain on a high fat diet and developed fatty liver and elevated lipid and insulin levels. Further genetic studies with mice carrying XO and XXY chromosome complements revealed that the differences between XX and XY mice are attributable to dosage of the X chromosome, rather than effects of the Y chromosome. A subset of genes that escape X chromosome inactivation exhibited higher expression levels in adipose tissue and liver of XX compared to XY mice, and may contribute to the sex differences in obesity. Overall, our study is the first to identify sex chromosome complement, a factor distinguishing all male and female cells, as a cause of sex differences in obesity and metabolism.
Differences exist between men and women in the development of obesity and related metabolic diseases such as type 2 diabetes and cardiovascular disease. Previous studies have focused on the sex-biasing role of hormones produced by male and female gonads, but these cannot account fully for the sex differences in metabolism. We discovered that removal of the gonads uncovers an important genetic determinant of sex differences in obesity—the presence of XX or XY sex chromosomes. We used a novel mouse model to tease apart the effects of male and female gonads from the effects of XX or XY chromosomes. Mice with XX sex chromosomes (relative to XY), regardless of their type of gonad, had increased body fat and ate more food during the sleep period. Mice with two X chromosomes also had accelerated weight gain, fatty liver, and hyperinsulinemia on a high fat diet. The higher expression levels of a subset of genes on the X chromosome that escape inactivation may influence adiposity and metabolic disease. The effect of X chromosome genes is present throughout life, but may become particularly significant with increases in longevity and extension of the period spent with reduced gonadal hormone levels.
Glioblastoma (GBM) is the most common malignant primary brain tumor of adults and one of the most lethal of all cancers. EGFR mutations (EGFRvIII) and PI3K hyperactivation are common in GBM, promoting tumor growth and survival, including through SREBP-1-dependent-lipogenesis. The role of cholesterol metabolism in GBM pathogenesis, its association with EGFR/PI3K signaling, and its potential therapeutic targetability are unknown. Here, studies in GBM cell lines, xenograft models and GBM clinical samples, including from patients treated with the EGFR tyrosine kinase inhibitor lapatinib, uncovered an EGFRvIII-activated, PI3K/SREBP-1-dependent tumor survival pathway through the LDL receptor. Targeting LDLR with the Liver X Receptor (LXR) agonist GW3965 caused IDOL (Inducible Degrader Of LDLR)-mediated LDLR degradation and increased expression of the ABCA1 cholesterol efflux transporter, potently promoting tumor cell death in an in vivo GBM model. These results demonstrate that EGFRvIII can promote tumor survival through PI3K-SREBP-1 dependent up-regulation of LDLR, and suggest a role for LXR agonists in the treatment of GBM patients.
Background & Aims
Liver X receptors (LXRs) are lipid-activated nuclear receptors with important roles in cholesterol transport, lipogenesis, and anti-inflammatory signaling. Hepatic stellate cells (HSCs) activate during chronic liver injury and mediate the fibrotic response. These cells are also major repositories for lipids, but the role of lipid metabolism during stellate cell activation remains unclear. Here we show that LXR signaling is an important determinant of stellate cell activation and susceptibility to fibrotic liver disease.
Immortalized and primary stellate cells purified from mice were treated with highly specific LXR ligands. Carbon tetrachloride (CCl4) and methionine choline deficiency (MCD) were used as chronic liver injury models. Reciprocal bone marrow transplants were performed to test the importance of hematopoietically-derived cells to the fibrotic response.
LXR ligands suppressed markers of fibrosis and stellate cell activation in primary mouse stellate cells. Lxrαβ −/− stellate cells produce increased levels of inflammatory mediators and conditioned media from Lxrαβ−/− cells increases the fibrogenic program of wild-type cells. Furthermore, Lxrαβ−/− stellate cells exhibit altered lipid morphology and increased expression of fibrogenic genes, suggesting they are primed for activation. In vivo, Lxrαβ−/− mice have marked susceptibility to fibrosis in two injury models. Bone marrow transplants point to altered stellate cell function, rather than hematopoietic cell inflammation, as the primary basis for the Lxrαβ−/− phenotype.
These results reveal an unexpected role for LXR signaling and lipid metabolism in the modulation of hepatic stellate cell function.
Nuclear receptors; LXRs; hepatic stellate cells; liver fibrosis
The most abundant immune cell type is the neutrophil, a key first responder after pathogen invasion. Neutrophil numbers in the periphery are tightly regulated to prevent opportunistic infections and aberrant inflammation. In healthy individuals, more than 1 × 109 neutrophils per kilogram body weight are released from the bone marrow every 24 hours. To maintain homeostatic levels, an equivalent number of senescent cells must be cleared from circulation. Recent studies indicate that clearance of senescent neutrophils by resident tissue macrophages and DCs helps to set homeostatic levels of neutrophils via effects on the IL-23/IL-17/G-CSF cytokine axis, which stimulates neutrophil production in the bone marrow. However, the molecular events in phagocytes underlying this feedback loop have remained indeterminate. Liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate both lipid metabolic and inflammatory gene expression. Here, we demonstrate that LXRs contribute to the control of neutrophil homeostasis. Using gain- and loss-of-function models, we found that LXR signaling regulated the efficient clearance of senescent neutrophils by peripheral tissue APCs in a Mer-dependent manner. Furthermore, activation of LXR by engulfed neutrophils directly repressed the IL-23/IL-17/G-CSF granulopoietic cytokine cascade. These results provide mechanistic insight into the molecular events orchestrating neutrophil homeostasis and advance our understanding of LXRs as integrators of phagocyte function, lipid metabolism, and cytokine gene expression.
Adult GPIHBP1-deficient mice (Gpihbp1−/−) have severe hypertriglyceridemia; however, the plasma triglyceride levels are only mildly elevated during the suckling phase when lipoprotein lipase (Lpl) is expressed at high levels in the liver. Lpl expression in the liver can be induced in adult mice with dietary cholesterol. We therefore hypothesized that plasma triglyceride levels in adult Gpihbp1−/− mice would be sensitive to cholesterol intake.
Methods and Results
After 4–8 weeks on a western diet containing 0.15% cholesterol, plasma triglyceride levels in Gpihbp1−/− mice were 10,000–12,000 mg/dl. When 0.005% ezetimibe was added to the diet to block cholesterol absorption, Lpl expression in the liver was reduced significantly, and the plasma triglyceride levels were significantly higher (>15,000 mg/dl). We also assessed plasma triglyceride levels in Gpihbp1−/− mice fed western diets containing either high (1.3%) or low (0.05%) amounts of cholesterol. The high-cholesterol diet significantly increased Lpl expression in the liver and lowered plasma triglyceride levels.
Treatment of Gpihbp1−/− mice with ezetimibe lowers Lpl expression in the liver and increases plasma triglyceride levels. A high-cholesterol diet had the opposite effects. Thus, cholesterol intake modulates plasma triglyceride levels in Gpihbp1−/− mice.
lipoprotein lipase; chylomicronemia; hypertriglyceridemia; GPIHBP1
PPARγ and Wnt signaling are central positive and negative regulators of adipogenesis, respectively. Here we identify the groucho family member TLE3 as a transcriptional integrator of the PPARγ and Wnt pathways. TLE3 is a direct target of PPARγ that participates in a feed-forward loop during adipocyte differentiation. TLE3 enhances PPARγ activity and functions synergistically with PPARγ on its target promoters to stimulate adipogenesis. At the same time, induction of TLE3 during differentiation provides a mechanism for termination of Wnt signaling. TLE3 antagonizes TCF4 activation by β-catenin in preadipocytes, thereby inhibiting Wnt target gene expression and reversing β-catenin-dependent repression of adipocyte gene expression. Transgenic expression of TLE3 in adipose tissue in vivo mimics the effects of PPARγ agonist and ameliorates high fat diet-induced insulin resistance. Our data suggest that TLE3 acts as a dual function switch, driving the formation of both active and repressive transcriptional complexes that facilitate the adipogenic program.
Atherosclerotic cardiovascular disease (ASCVD) affects more than 1 in 3 American adults. Hypercholesterolemia is a major treatable risk factor for ASCVD, yet many individuals fail to reach target levels of LDL-cholesterol (LDL-C) through the use of statins and lifestyle changes. The E3 ubiquitin ligase myosin regulatory light chain–interacting protein (MYLIP; also known as IDOL) is a recently identified regulator of the LDL receptor (LDLR) pathway. Genome-wide association studies (GWASs) in populations of mixed European descent have identified noncoding variants in the MYLIP region as being associated with LDL-C levels, but no underlying functional variants were pinpointed. In order to fine-map actual susceptibility variants, we studied a population demographically distinct from the discovery population to ensure a different pattern of linkage disequilibrium. Our analysis revealed that in a Mexican population, the nonsynonymous SNP rs9370867, which encodes the N342S amino acid substitution, is an underlying functional variant that was associated with high total cholesterol and accounted for one of the previous significant GWAS signals. Functional characterization showed that the Asn-encoding allele was associated with more potent LDLR degradation and decreased LDL uptake. Mutagenesis of residue 342 failed to affect intrinsic MYLIP E3 ligase activity, but it was critical for LDLR targeting. Our findings suggest that modulation of MYLIP activity can affect LDL-C levels and that pharmacologic inhibition of MYLIP activity might be a useful strategy in the treatment of dyslipidemia and ASCVD.
The mechanisms that direct cell-type specific PPAR gene programs are poorly understood. In this issue of Immunity, Szanto et al identify STAT6 as a transcriptional switch that licenses PPARγ-dependent gene expression in macrophages and dendritic cells.
Previously, we identified the E3 ubiquitin ligase Idol (inducible degrader of the low-density lipoprotein [LDL] receptor [LDLR]) as a posttranscriptional regulator of the LDLR pathway. Idol stimulates LDLR degradation through ubiquitination of its C-terminal domain, thereby limiting cholesterol uptake. Here we report the generation and characterization of mouse embryonic stem cells homozygous for a null mutation in the Idol gene. Cells lacking Idol exhibit markedly elevated levels of the LDLR protein and increased rates of LDL uptake. Furthermore, despite an intact sterol responsive element-binding protein (SREBP) pathway, Idol-null cells exhibit an altered response to multiple regulators of sterol metabolism, including serum, oxysterols, and synthetic liver X receptor (LXR) agonists. The ability of oxysterols and lipoprotein-containing serum to suppress LDLR protein levels is reduced, and the time course of suppression is delayed, in cells lacking Idol. LXR ligands have no effect on LDLR levels in Idol-null cells, indicating that Idol is required for LXR-dependent inhibition of the LDLR pathway. In line with these results, the half-life of the LDLR protein is prolonged in the absence of Idol. Finally, the ability of statins and PCSK9 to alter LDLR levels is independent of, and additive with, the LXR-Idol pathway. These results demonstrate that the LXR-Idol pathway is an important contributor to feedback inhibition of the LDLR by sterols and a biological determinant of cellular LDL uptake.
Ligand activation of liver X receptors (LXRs) has been shown to impact both lipid metabolism and inflammation. One complicating factor in studies utilizing synthetic LXR agonists is the potential for pharmacologic and receptor-independent effects. Here, we describe an LXR gain-of-function system that does not depend on the addition of exogenous ligand. We generated transgenic mice expressing a constitutively active VP16-LXRα protein from the aP2 promoter. These mice exhibit increased LXR signaling selectively in adipose and macrophages. Analysis of gene expression in primary macrophages derived from two independent VP16-LXRα transgenic lines confirmed the ability of LXR to drive expression of genes involved in cholesterol efflux and fatty acid synthesis. Moreover, VP16-LXRα expression also suppressed the induction of inflammatory genes by lipopolysaccharide to a comparable degree as synthetic agonist. We further utilized VP16-LXRα-expressing macrophages to identify and validate new targets for LXRs, including the gene encoding ADP-ribosylation factor-like 7 (ARL7). ARL7 has previously been shown to transport cholesterol to the membrane for ABCA1-associated removal and thus may be integral to the LXR-dependent efflux pathway. We show that the ARL7 promoter contains a functional LXRE and can be transactivated by LXRs in a sequence-specific manner, indicating that ARL7 is a direct target of LXR. These findings provide further support for an important role of LXRs in the coordinated regulation of lipid metabolic and inflammatory gene programs in macrophages.
ADP-ribosylation factor-like 7; liver X receptor; nuclear receptor; macrophage; inflammation; lipid metabolism
First discovered as orphan receptors, liver X receptors (LXRs) were subsequently identified as the nuclear receptor target of the cholesterol metabolites, oxysterols.1 There are 2 LXR receptors encoded by distinct genes: LXRα is most highly expressed in the liver, adipose, kidney, adrenal tissues and macrophages, and LXRβ is ubiquitously expressed. Despite differential tissue distribution, these isoforms have 78% homology in their ligand-binding domain and appear to respond to the same endogenous ligands. Work over the past 10 years has shown that the LXR pathway regulates lipid metabolism and inflammation via both the induction and repression of target genes. Given the importance of cholesterol regulation and inflammation in the development of cardiovascular disease, it is not surprising that activation of the LXR pathway attenuates various mechanisms underlying atherosclerotic plaque development.2 In this minireview we will discuss the impact of the LXR pathway on both cholesterol metabolism and atherosclerosis.
Nur77 is an orphan nuclear receptor with pleotropic functions. Previous studies have identified Nur77 as a transcriptional regulator of glucose utilization genes in skeletal muscle and gluconeogenesis in liver. However, the net functional impact of these pathways is unknown. To examine the consequence of Nur77 signaling for glucose metabolism in vivo, we challenged Nur77 null mice with high-fat feeding.
RESEARCH DESIGN AND METHODS
Wild-type and Nur77 null mice were fed a high-fat diet (60% calories from fat) for 3 months. We determined glucose tolerance, tissue-specific insulin sensitivity, oxygen consumption, muscle and liver lipid content, muscle insulin signaling, and expression of glucose and lipid metabolism genes.
Mice with genetic deletion of Nur77 exhibited increased susceptibility to diet-induced obesity and insulin resistance. Hyperinsulinemic-euglycemic clamp studies revealed greater high-fat diet–induced insulin resistance in both skeletal muscle and liver of Nur77 null mice compared with controls. Loss of Nur77 expression in skeletal muscle impaired insulin signaling and markedly reduced GLUT4 protein expression. Muscles lacking Nur77 also exhibited increased triglyceride content and accumulation of multiple even-chained acylcarnitine species. In the liver, Nur77 deletion led to hepatic steatosis and enhanced expression of lipogenic genes, likely reflecting the lipogenic effect of hyperinsulinemia.
Collectively, these data demonstrate that loss of Nur77 influences systemic glucose metabolism and highlight the physiological contribution of muscle Nur77 to this regulatory pathway.
The liver X receptors (LXRs) are a family of nuclear receptor transcription factors that are activated by oxysterols and have defined roles in both lipid metabolism and cholesterol regulation. LXRs also affect antimicrobial responses and have anti-inflammatory effects in macrophages. As mice lacking LXRs are more susceptible to infection by intracellular bacteria Listeria monocytogenes and Mycobacterium tuberculosis, we hypothesized that LXR might also influence macrophage responses to the intracellular protozoan parasite Leishmania chagasi/infantum, a causative agent of visceral leishmaniasis.
Methods and Findings
Surprisingly, both LXRα knock-out and LXRα/LXRβ double-knock-out (DKO) mice were markedly resistant to systemic L. chagasi/infantum infection compared to wild-type mice. Parasite loads in the livers and spleens of these animals were significantly lower than in wild-type mice 28 days after challenge. Bone marrow-derived macrophages from LXR-DKO mice infected with L. chagasi/infantum in vitro in the presence of IFN-γ were able to kill parasites more efficiently than wild-type macrophages. This enhanced killing by LXR-deficient macrophages correlated with higher levels of nitric oxide produced, as well as increased gene expression of IL-1β. Additionally, LXR ligands abrogated nitric oxide production in wild-type macrophages in response to infection.
These observations suggest that LXR-deficient mice and macrophages mount antimicrobial responses to Leishmania infection that are distinct from those mounted by wild-type mice and macrophages. Furthermore, comparison of these findings to other intracellular infection models suggests that LXR signaling pathways modulate host antimicrobial responses in a complex and pathogen-specific manner. The LXR pathway thus represents a potential therapeutic target for modulating immunity against Leishmania or other intracellular parasites.
Leishmania spp. are protozoan single-cell parasites that are transmitted to humans by the bite of an infected sand fly, and can cause a wide spectrum of disease, ranging from self-healing skin lesions to potentially fatal systemic infections. Certain species of Leishmania that cause visceral (systemic) disease are a source of significant mortality worldwide. Here, we use a mouse model of visceral Leishmania infection to investigate the effect of a host gene called LXR. The LXRs have demonstrated important functions in both cholesterol regulation and inflammation. These processes, in turn, are closely related to lipid metabolism and the development of atherosclerosis. LXRs have also previously been shown to be involved in protection against other intracellular pathogens that infect macrophages, including certain bacteria. We demonstrate here that LXR is involved in susceptibility to Leishmania, as animals deficient in the LXR gene are much more resistant to infection with the parasite. We also demonstrate that macrophages lacking LXR kill parasites more readily, and make higher levels of nitric oxide (an antimicrobial mediator) and IL-1β (an inflammatory cytokine) in response to Leishmania infection. These results could have important implications in designing therapeutics against this deadly pathogen, as well as other intracellular microbial pathogens.
We have previously shown that mouse atherosclerosis regression involves monocyte-derived (CD68+) cell emigration from plaques and is dependent on the chemokine receptor CCR7. Concurrent with regression, mRNA levels of the gene encoding LXRα are increased in plaque CD68+ cells, suggestive of a functional relationship between LXR and CCR7. To extend these results, atherosclerotic Apoe–/– mice sufficient or deficient in CCR7 were treated with an LXR agonist, resulting in a CCR7-dependent decrease in plaque CD68+ cells. To test the requirement for LXR for CCR7-dependent regression, we transplanted aortic arches from atherosclerotic Apoe–/– mice, or from Apoe–/– mice with BM deficiency of LXRα or LXRβ, into WT recipients. Plaques from both LXRα- and LXRβ-deficient Apoe–/– mice exhibited impaired regression. In addition, the CD68+ cells displayed reduced emigration and CCR7 expression. Using an immature DC line, we found that LXR agonist treatment increased Ccr7 mRNA levels. This increase was blunted when LXRα and LXRβ levels were reduced by siRNAs. Moreover, LXR agonist treatment of primary human immature DCs resulted in functionally significant upregulation of CCR7. We conclude that LXR is required for maximal effects on plaque CD68+ cell expression of CCR7 and monocyte-derived cell egress during atherosclerosis regression in mice.
We previously described the use of a cell-based screening approach to identify small molecules that regulate adipocyte differentiation. Here we identify the amiloride derivative phenamil as an adipogenic compound. Phenamil acutely induces expression of the key transcription factor of adipogenesis, peroxisome proliferator-activated receptor γ (PPARγ) and, consequently, promotes the differentiation of multiple preadipocyte cell lines, including 3T3-L1 and F442A. Interestingly, the adipogenic action of phenamil is distinct from and additive with both PPARγ ligands and the previously identified adipogenic small molecule harmine. To identify signaling pathways mediating phenamil's effects, we performed transcriptional profiling of 3T3-F442A preadipocytes. ETS variant 4 (ETV4) was identified as a gene rapidly induced by phenamil but not by other adipogenic small molecules or PPARγ agonists. Transient expression of ETV4 in preadipocytes enhances the expression of PPARγ. Stable overexpression of ETV4 promotes expression of PPARγ and its downstream target genes and enhances morphological differentiation. Finally, knockdown of PPARγ expression by shRNA blocks the effects of phenamil on adipocyte differentiation and gene expression, but it does not block phenamil induction of ETV4, which suggests that ETV4 acts upstream of PPARγ in differentiation processes. These results identify a phenamil as new small molecule tool for the probing of adipocyte differentiation that acts, at least in part, through induction of ETV4 expression.
ETS variant; peroxisome proliferator-activated receptor; diabetes; adipose tissue; fatty acid; nuclear receptor
We have previously identified the E3 ubiquitin ligase-inducible degrader of the low density lipoprotein receptor (LDLR) (Idol) as a post-translational modulator of LDLR levels. Idol is a direct target for regulation by liver X receptors (LXRs), and its expression is responsive to cellular sterol status independent of the sterol-response element-binding proteins. Here we demonstrate that Idol also targets two closely related LDLR family members, VLDLR and ApoE receptor 2 (ApoER2), proteins implicated in both neuronal development and lipid metabolism. Idol triggers ubiquitination of the VLDLR and ApoER2 on their cytoplasmic tails, leading to their degradation. We further show that the level of endogenous VLDLR is sensitive to cellular sterol content, Idol expression, and activation of the LXR pathway. Pharmacological activation of the LXR pathway in mice leads to increased Idol expression and to decreased Vldlr levels in vivo. Finally, we establish an unexpected functional link between LXR and Reelin signaling. We demonstrate that LXR activation results in decreased Reelin binding to VLDLR and reduced Dab1 phosphorylation. The identification of VLDLR and ApoER2 as Idol targets suggests potential roles for this LXR-inducible E3 ligase in the central nervous system in addition to lipid metabolism.
Cholesterol Metabolism; Lipoprotein Receptor; Metabolism; Nuclear Receptors; Ubiquitin Ligase; LXR
We examined the effect of LXR agonists on vascular calcification, prevalent in atherosclerotic lesions. T0901317, an LXR agonist, augmented protein kinase A (PKA)-induced mineralization and alkaline phosphatase (ALP) activity in aortic smooth muscle cells isolated from wild-type, but not from Lxrβ-/- mice. A six-hour T0901317 treatment augmented the PKA-induced expression of the phosphate transporter Pit-1, a positive regulator of mineralization, suggesting a direct role. A ten-day T0901317 treatment attenuated PKA-induced expression of mineralization inhibitors, osteopontin and ectonucleotide pyrophosphatase/phosphodiesterase-1, suggesting an indirect role. The effects of T0901317 were attenuated by inhibition of ALP, Pit-1 and Rho-associated kinase, but not by inhibition of PKA. These results suggest that T0901317-augmented mineralization occurs downstream of PKA, involving both direct and indirect LXR-mediated pathways.
T0901317; LXR; calcification; smooth muscle cells
Effective clearance of apoptotic cells by macrophages is essential for immune homeostasis. The transcriptional pathways that allow macrophages to sense and respond to apoptotic cells are poorly defined. We demonstrate here that LXR signaling is important for both apoptotic cell clearance and the maintenance of immune tolerance. Apoptotic cell engulfment activates LXR and thereby induces the expression of Mer, a receptor tyrosine kinase critical for phagocytosis. LXR null macrophages exhibit a selective defect in phagocytosis of apoptotic cells and an aberrant pro-inflammatory response to them. As a consequence of these defects, mice lacking LXRs manifest a breakdown in self-tolerance and develop autoantibodies and autoimmune glomerulonephritis. Treatment with an LXR agonist ameliorates disease progression in mouse models of Lupus-like autoimmunity. Thus, activation of LXR by apoptotic cells engages a virtuous cycle that promotes their own clearance and couples engulfment to the suppression of inflammatory pathways.
Cellular cholesterol levels reflect a balance between uptake, efflux and endogenous synthesis. Here we show that the sterol-responsive nuclear receptor LXR helps maintain cholesterol homeostasis not only through promotion of cholesterol efflux, but also through suppression of low-density lipoprotein (LDL) uptake. LXR inhibits the LDL receptor (LDLR) pathway through transcriptional induction of Idol (Inducible Degrader of the LDLR), an E3 ubiquitin ligase that triggers ubiquitination of the LDLR on its cytoplasmic domain, thereby targeting it for degradation. LXR ligand reduces, whereas LXR knockout increases, LDLR protein levels in vivo in a tissue-selective manner. Idol knockdown in hepatocytes increases LDLR protein levels and promotes LDL uptake. Conversely, adenovirus-mediated expression of Idol in mouse liver promotes LDLR degradation and elevates plasma LDL levels. The LXR-Idol-LDLR axis defines a complementary pathway to sterol response element binding proteins for sterol regulation of cholesterol uptake.
Stimulation of osteoblast differentiation from mesenchymal stem cells is a potential strategy for bone repair. Bone morphogenetic proteins (BMPs) that induce osteoblastic differentiation have been successfully used in humans to treat fractures. Here we outline a new approach to the stimulation of osteoblast differentiation using small molecules that stimulate BMP activity. We have identified the amiloride derivative phenamil as a stimulator of osteoblast differentiation and mineralization. Remarkably, phenamil acts cooperatively with BMPs to induce the expression of BMP target genes, osteogenic markers, and matrix mineralization in both mesenchymal stem cell lines and calvarial organ cultures. Transcriptional profiling of cells treated with phenamil led to the identification of tribbles homolog 3 (Trb3) as a mediator of its effects. Trb3 is induced by phenamil selectively in cells with osteoblastic potential. Both Trb3 and phenamil stabilize the expression of SMAD, the critical transcription factor in BMP signaling, by promoting the degradation of SMAD ubiquitin regulatory factor 1. Small interfering RNA-mediated knockdown of Trb3 blunts the effects of phenamil on BMP signaling and osteogenesis. Thus, phenamil induces osteogenic differentiation, at least in part, through Trb3-dependent promotion of BMP action. The synergistic use of small molecules such as phenamil along with BMPs may provide new strategies for the promotion of bone healing.
Experimental and clinical studies link Chlamydia pneumoniae infection to atherogenesis and athero-thrombotic events, but the underlying mechanisms are unclear. We tested the hypothesis that C. pneumoniae-induced acceleration of atherosclerosis in ApoE−/− mice is reciprocally modulated by activation of TLR-mediated innate immune or LXRα signaling pathways. We infected ApoE−/− mice and ApoE−/− mice that also lacked TLR2 or TLR4 or MyD88 or LXRα intranasally with C. pneumoniae followed by high-fat diet feeding for 4 months. Mock infected littermates served as controls. Atherosclerosis was assessed in aortic sinuses and in en face preparation of whole aorta. The numbers of activated dendritic cells (DCs) within plaques, and serum levels of cholesterol and proinflammatory cytokines were also measured. C. pneumoniae infection markedly accelerated atherosclerosis in ApoE deficient mice that was associated with increased numbers of activated DCs in aortic sinus plaques and higher circulating levels of MCP-1, IL-12p40, IL-6 and TNF-α. In contrast, C. pneumoniae infection had only a minimal effect on atherosclerosis, accumulation of activated DCs in the sinus plaques, or circulating cytokine increases in ApoE−/− mice that were also deficient in either TLR2, TLR4, or MyD88. However, C. pneumoniae-induced acceleration of atherosclerosis in ApoE−/− mice was further enhanced in ApoE−/−/LXRα−/− double knockout mice, and was accompanied by higher serum levels of IL-6 and TNF-α. We conclude that C. pneumoniae-infection accelerates atherosclerosis in hypercholesterolemic mice predominantly through a TLR/MyD88-dependent mechanism, and that LXRα appears to reciprocally modulate and reduce the pro-atherogenic effects of C. pneumoniae infection.
Biological systems are integrated networks constantly responding to internal and external stimulators. Understanding the intrinsic response to an imbalanced system provides the opportunity to develop therapeutic approaches to reinstate the natural balanced state. Increasing evidence suggests that members of the nuclear receptor superfamily integrate both inflammatory and metabolic signals to maintain homeostasis in immune cells such as macrophages and lymphocytes. PPAR and LXR are nuclear receptors activated by fatty acid and cholesterol derivatives respectively that control the expression of an array of genes involved in lipid metabolism and inflammation. Recent studies have uncovered distinct mechanisms for transcriptional regulation of metabolic and inflammatory target genes by PPAR and LXR and have expanded the biology of these receptors to include roles in alternative macrophage activation and adaptive immunity.